Photograph of client using tissue to look for air conditioning air flow

Air Conditioning / Heat Pump Repair FAQ: How to Diagnose & Repair Air Conditioners & Heat Pumps
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Detailed air conditioner diagnosis & repair checklist
How to diagnose & repair most air conditioning system problems
Air conditioner / heat pump compressor defect diagnosis - list of key diagnostic procedures & articles
Questions & Answers about air conditioner or heat pump troubleshooting and repair

This Air Conditioning Troubleshooting & Diagnosis FAQ (list of frequently asked questions about air conditioners) lists the major areas of investigation to be followed in diagnosing all types of problems with air conditioning systems and heat pump equipment.

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Typical air conditioner concerns include compressor or blower assembly noises, failure of air conditioners to start or to keep running, inadequate cool air flow or air quantity, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely. Also discussed are various electrical wiring, fusing, overcurrent, and thermostat or control defects on air conditioners.

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Here are the Major Areas to Investigate for Problems With Air Conditioning Systems & Equipment

SYSTEM OPERATION what are the basics?

OPERATING CONTROLS what and where are the various air conditioner controls & switches?
SAFETY CONTROLS what safety controls and switches are required on air conditioners?

OPERATING TEMPERATURES how & where to measure central air conditioning temperatures
OPERATING DEFECTS

The air conditioning system will not operate at all: power may be off, controls may be improperly set, or the system may be inoperative. (Confirm that the system has electrical power and that all of its control switches and thermostat are set to on and cooling positions.
see CONTROLS & SWITCHES
The air conditioning system operates but does not produce cool air, or not enough cool air.
see LOST COOLING CAPACITY
The air conditioning system operates but there is not enough cool air flow at the registers.
see DUCT SYSTEM DEFECTS.
The compressor was short-cycling, that is, turning itself on and off rapidly, perhaps every few seconds or minutes rather than producing a normal on-cycle of 10 minutes or longer.
see COMPRESSOR CONDENSER
The compressor was noisy, during startup, indicating an operating problem.
see COMPRESSOR NOISES.
The compressor squealed at startup, indicating that service may be needed promptly to prevent possibly costly damage to the system. This repair/maintenance item should not be deferred.
see COMPRESSOR NOISES.
Inside cooling or evaporator coil defects that can be seen by eye: dirty coil, blocked coil, frost on the coil, improperly sized evaporator coil, improper evaporator or cooling coil placement in the system.
see AIR HANDLER / BLOWER UNITS.
Air flow across the air conditioning evaporator coil: if airflow is weak for any reason (dirty coil, duct system defects, blower fan defects, dirty blower squirrel cage fan), the air conditioning system will not operate properly.

Some experts write that there should be between 350 and 400 cubic feet of air per minute (CFM) moving across the evaporator (cooling) coil for each ton of air conditioner capacity.

One ton = 12,000 BTUH so if your AC unit is a 24,000 BTUH unit it is a "two ton" unit and needs to see 700 to 800 CFM of air across the evaporator coil.

Some home inspectors and air conditioning service technicians carry a small airflow meter that can actually measure this number with fair accuracy.

The same tool is nice for comparing air flow and balancing air flow at various building supply ducts and registers.

See COOLING COIL or EVAPORATOR COIL and also DIRTY COOLING COIL - how dirt and debris appear, what to do about a dirty evaporator coil.

Evaporator coil cleaning often requires cutting refrigerant lines, removal of the coil and other components for cleaning, and reinstallation, pulling a vacuum on the refrigerant lines, and recharge with refrigerant. Such service and repair may involve significant expense, although there are some "in place" cleaning methods using foams and sprays that are a simpler procedure.
See DIRTY COIL CLEANING PROCEDURES, FROST BUILD-UP on AIR CONDITIONER COILS and OPERATING TEMPERATURES.
Dirty air conditioning filters, - a source of increased operating costs. Very dirty filters can eventually block the fan itself, leading to more costly repairs. The filters should be changed monthly when the system is in use. Failure to properly filter dust from the return air supply can load the fan or evaporator coil with dust and prevent proper system operation. Installing a filter is normally a minor expense. Dirty, blocked evaporator coils can result in improper system operation, very low output temperatures, low air flow (compensated sometimes by increased fan speed), and sometimes frost on the coil and failure of system components.
See AIR FILTERS for HVAC SYSTEMS, and also AIR HANDLER / BLOWER UNITS. See OPERATING TEMPERATURES.
The air conditioner operates but our electrical bills are too high.
See DIRTY COIL CLEANING PROCEDURES, SEER RATINGS and see DUCT SYSTEM DEFECTS.
The air conditioner fuse keeps blowing or the circuit breaker trips: could mean a compressor which needs repair or replacement, or it could mean a problem with the unit's electrical wiring, such as use of aluminum wiring combined with bad electrical connections. Have your service technician check the condition of the electrical circuit first (that's cheaper than a new air conditioning compressor) and then check the condition of the compressor.
see COMPRESSOR CONDENSER for air conditioner compressor diagnosis and repair; and if your air conditioning system electrical wiring is aluminum, review the aluminum wiring hazards at ALUMINUM WIRING HAZARDS & REPAIRS since a serious fire hazard could be present.

LOST COOLING CAPACITY explains how to diagnose & repair an Air Conditioner system which has lost cooling capacity
COMPRESSOR CONDENSER which describes problems with air conditioner compressor/condenser units

INSTALLATION ERRORS
CONDENSING COIL REPAIR REPLACE
COMPRESSOR NOISES - diagnosing and curing a noisy air conditioner compressor unit
HARD STARTING - how to fix an air conditioner compressor motor that won't start
TIGHT or SEIZED AC COMPRESSORS - how to diagnose a compressor that needs replacement
COMPRESSOR PRESSURE READINGS - what are the usual system pressures in an air conditioner or heat pump?
BURNED-OUT COMPRESSOR - how we diagnose a compressor motor that is burned out
REPLACING A COMPRESSOR - steps in installing a new air conditioner compressor
A/C CAPACITORS - using starter capacitors on an air conditioner compressor or other electric motors

AIR HANDLER / BLOWER UNITS which describes problems with the air handler, air filters, and the cooling coil itself

DIRTY A/C BLOWERS

AIR FILTERS for HVAC SYSTEMS - air filters are critical not only for building health and comfort but to keep an adequate air volume moving through the air conditioning or heat pump system

Air Filter Location - where should air filters be located
Dirty Air Filter Problems - what happens when the air filter gets dirty - how often should filters be changed?
Missing or Leaky Air Filters - what happens if we simply omit the air filter - some costly problems develop

AIR FILTERS, OPTIMUM INDOOR - what is the absolute best, most effective way to clean indoor air

Air Filter Efficiency - MERV and other air filter ratings
Optimum Air Filter Design / Location
How to Cascade Air Filters

AIR FILTER EFFECTIVENESS - what determines how effective an air filter will be

FIBERGLASS & AIR FILTERS - how these filters work

Air Filters Shed Fibers? - are there problems with air contamination?
Fiberglass & Test Lab Accuracy
Variations in Fiber Size

SOURCES FOR AIR FILTERS - where to obtain air filters of various sorts

OTHER AIR CLEANERS - how effective are other air cleaners such as free-standing air purifier units? Are some of them dangerous? Maybe. Are claims exaggerated? Usually.

CLEANING & Legionella BACTERIA how to remove or prevent bacterial growth in air conditioning systems, cleaning condensate trays, cooling towers, use of chlorine, bromine, disinfectants in air conditioners
CONDENSATE HANDLING

COOLING COIL or EVAPORATOR COIL - problems at the evaporator coil can cause total loss of cooling capacity (or heating with a heat pump)
- DAMAGED COOLING COIL
- DIRTY COOLING COIL - how dirt and debris appear, what to do about a dirty evaporator coil
- DIRTY COIL CLEANING PROCEDURES - a review of methods for cleaning a dirty A/C coil
- FROST BUILD-UP on AIR CONDITIONER COILS - the cause and cure of ice on the A/C cooling coil
DUCT SYSTEM DEFECTS which describes problems with the duct system.

SUPPLY DUCTS & REGISTERS, how to inspect for defects
RETURN AIR REGISTERS & DUCTS , how to inspect for defects
BALANCING AIR DUCT FLOW, how to balance the air flow in a heating or cooling system
UNDERSIZED RETURN DUCTS, how to recognize under-sized air conditioning ducts or inadequate return air
INCREASING RETURN AIR, poor air flow cause and cure, how to increase return air and thus cooling air flow
LOCATION OF REGISTERS & DUCTS, improper placement can reduce air conditioning capacity or may be unsafe
DUCT ROUTING & SUPPORT errors in duct routing and support can cause poor air flow or air quality and mold problems
WET CORRODED DUCT WORK, water and rust as indicators of duct conditions, possible mold or other pathogenic health hazards
LEAKY DUCT CONNECTIONS, how to diagnose and correct leaky ducts
VIBRATION DAMPENERS and asbestos hazards at the air conditioner air handling unit or blower
GOODMAN GRAY FLEXDUCT deterioration of flex duct, cause, cure
OWENS CORNING FLEXDUCT deterioration of flex duct, cause, cure
UNSAFE DUCT OPENINGS as a possible source of air quality issues or carbon monoxide poisoning hazards
ASBESTOS HVAC DUCTS how to recognize asbestos materials on heating and cooling air ducts and equipment
TRANSITE PIPE AIR DUCTS as a possible source of asbestos, blocked ducts, reduced air flow, mold
FIBERGLASS HVAC DUCTS fiberglass contamination of indoor air, fiberglass as a possible mold source

REFRIGERANTS in AIR CONDITIONERS & HEAT PUMPS:lists historic and currently in-use air conditioning refrigerants and gives their characteristics
- REFRIGERANT LEAK DETECTION - how to find where refrigerant gas leaks are happening
- REFRIGERANT LEAK REPAIR - how to fix air conditioning or heat pump refrigerant gas leaks

How to diagnose and fix an air conditioning system that is not working

Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING & HEAT PUMP SYSTEMS.

If your air conditioning or heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.

A/C - HEAT PUMP CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
LOST COOLING CAPACITY: what to do when not enough cool air comes out of the system
- What to check first if there is no cool air or not enough cool air
- Compressor failure diagnosis: basic checks of the air conditioner compressor
- Ducts & Air Handler diagnosis: basic checks of the indoor air handler (blower), air ducts, and filter systems
COMPRESSOR CONDENSER: problems with air conditioner compressor/condenser units. Also see FAN, COMPRESSOR/CONDENSER UNIT for help in diagnosing and fixing problems with the outdoor compressor/condenser fan and fan motor.
AIR HANDLER UNIT: problems with the air handler, air filters, and the cooling coil itself
DUCT SYSTEM DEFECTS: problems with the air duct system, air filters, supply registers, return air registers
A/C REFRIGERANT LEAK DETECTION: how to use a TIF5000 to detect air conditioning refrigerant gas leak
A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair

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AIR CONDITIONING & HEAT PUMP SYSTEMS

A/C - HEAT PUMP CONTROLS & SWITCHES
A/C DATA TAGS
A/C - HEAT PUMP CRITICAL DEFECTS
A/C DIAGNOSTIC FAQs
A/C REFRIGERANTS
A/C TYPES, ENERGY SOURCE
AGE of AIR CONDITIONERS & HEAT PUMPS

AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER / BLOWER UNITS
APPLIANCE EFFICIENCY RATINGS

BACKUP HEAT for HEAT PUMPS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books

CAPACITORS for HARD STARTING MOTORS
CAPILLARY TUBES
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
COMBUSTION GASES & PARTICLE HAZARDS

COMPRESSOR & CONDENSING COIL, A/C

CONDENSATE HANDLING, A/C
CONDENSATION or SWEATING PIPES, TANKS
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
COOLING LOAD REDUCTION by ROOF VENTS
COMBUSTION GASES & PARTICLE HAZARDS
CONDENSATE HANDLING, A/C
CONDENSATION or SWEATING PIPES, TANKS

DATA TAGS on AIR CONDITIONERS
DEFINITION of Heating & Cooling Terms
DEHUMIDIFICATION PROBLEMS
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DUCT SYSTEM & DUCT DEFECTS
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST CONTAMINATION FROM HVAC?

EDUCATION, HVAC SCHOOLS
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT
EVAPORATIVE COOLING SYSTEMS
EVAPORATOR COIL or COOLING COIL
EXPANSION VALVES, REFRIGERANT

FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FAN NOISES

FURNACES WARM AIR HEATING SYSTEMS

GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION & MEASUREMENT
GAUGE, REFRIGERATION PRESSURE TEST

HEAT LOSS (or GAIN) in buildings
HEAT PUMPS
HUMIDITY LEVEL TARGET

INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS

LOST COOLING CAPACITY

MANUALS & PARTS GUIDES - HVAC
MOTOR OVERLOAD RESET SWITCH

NOISE AIR CONDITIONER / HEAT PUMP
NOISE / SOUND DIAGNOSIS & CURE
  Air Leak Noises
  AIR CONDITIONING & HEAT PUMP NOISES

OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES

PORTABLE ROOM AIR CONDITIONERS
PRESSURE READINGS, REFRIGERANT

REPAIR GUIDE, AIR CONDITIONERS / HEAT PUMPS
REPAIR & DIAGNOSTIC FAQs for A/C

REFRIGERANTS
  GAUGE, REFRIGERATION PRESSURE TEST
  REFRIGERANT CHARGING PROCEDURE
  REFRIGERANT DRIERS & FILTERS
  REFRIGERANT LEAK DETECTION
  REFRIGERANT LEAK REPAIR
  REFRIGERANT METERING DEVICES TEVs
  REFRIGERANT METERING CAPILLARY TUBES
  REFRIGERANT PIPING & DISTANCES
  REFRIGERANT PRESSURE READINGS
RETROFIT SIZING for A/C or HEAT PUMPS

SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
SWAMP COOLERS

THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
Asbestos HVAC Ducts and Flues field identification photos and guide
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
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Here are the Major Areas to Investigate for Problems With Air Conditioning Systems & Equipment

How to diagnose and fix an air conditioning system that is not working

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair